There has been known friction stir welding (FSW) as the technique of welding workpieces by rotating and pressing a cylindrical tool (having a probe projecting from the center of the distal end of a shoulder portion) to a joining line along which the workpieces each made of a metal material are butted against each other and relatively shifting the tool along the joining line so as to soften the workpieces with use of generated frictional heat. Furthermore, friction stir processing (FSP) of improving strength and hardness of the surface of a workpiece, and friction spot joining (FSJ) of performing spot joining of workpieces are also performed with use of the tool. The FSW, the FSP, and the FSJ are collectively referred to as friction stir processing.
In order to finish a processed portion of a workpiece with excellent quality in friction stir processing, it is necessary to keep, within an appropriate range, height relationship between the tool and the workpiece, in particular, a gap S1 between the distal end of the probe of the tool and the rear surface of the workpiece and a buried amount S2 of the shoulder portion of the tool into the workpiece (see FIG. 5).
There is a conventional friction stir welding apparatus for performing load control on a tool so as to substantially constantly keep pressurizing force of the tool to a workpiece (Patent Document 1). More specifically, as shown in FIG. 8, in the conventional friction stir welding apparatus, an air cylinder 640 biases downward a processing head 620 retaining a tool 610 so as to press the tool 610 against a workpiece 660 and bring the tool 610 into contact with the workpiece 660. A pressure reducing valve 650 of constant secondary pressure controls to substantially constantly keep bias force of the tool 610 generated by the air cylinder 640 to the workpiece 660. In the conventional friction stir welding apparatus, the bias force of the air cylinder 640 is kept substantially constantly to substantially constantly keep the pressurizing force of the tool 610 to the workpiece 660. It is possible to substantially constantly keep the height relationship between the tool 610 and the workpiece 660 even when there is variation in thickness of the workpiece 660 due to a production error, an error in setting on a surface plate 670, local deformation of the surface of the surface plate 670, or the like. It is accordingly possible to achieve excellent welding quality of the workpiece 660.